An ECM apparatus that is described in U.S. Pat. No. 7,479,214 includes a stationary cathode tool having a passage, and a drive mechanism for moving a bar-shaped workpiece through the passage of the cathode tool while simultaneously rotating the workpiece. Electrolyte flows, from a manifold on one end of the cathodic tool to a manifold at the other end, through the passage, between the wall of the passage and the workpiece. An electric current is simultaneously established in the electrolyte, between the wall of the passage and the workpiece. The internal shape of the cathodic tool wall has a gradual transition from a circular entry opening to a lobed exit opening, and lobes formed in the wall of the tool are shaped so that they twist in the direction of workpiece rotation, in order to form helical lobes in the workpiece.
The issue with this design is the high power consumption that is directly related to the amount of metal that the process has to dissolve to get the finished shape for the workpiece from the initial blank. By making use of the teaching of U.S. Pat. No. 7,479,214, all the metal outside the final shape has to be dissolved. The need to dissolve this much metal not only causes the high power requirements but also affects the speed at which the workpiece can be advanced through the electrode.
The present invention uses a similar ECM process as taught by U.S. Pat. No. 7,479,214 but reduces the power requirements in a variety of ways. The principle way this result is achieved is to use an electrode that has a much smaller thickness than the maximum thickness of the part that is to be cut away from the workpiece. The electrode can be shaped in various ways including open shapes or closed shapes like rings, for example. The electrode might be hollow or might have at least passages through which the electrolyte is delivered through openings in the electrode that can be at a leading end in the direction of electrode movement with respect to the workpiece to be machined. Alternatively, the outlets for the electrolyte can be on or in or near other surfaces of the electrode or the electrolyte might be directed with seals or other appropriate means to a part of the electrode for dissolving and removing a narrow band of metal to cut into the workpiece. The electrode can be rotated or otherwise moved with respect to the workpiece to create a desired shape of the cut as it is axially advanced, e.g. to create a desired pitch. The electrode can have lobe shapes which might be already inclined for a desired pitch. By using an electrode with lobe shapes, it is a function of controlling the axial speed and rate of rotation to get the desired lobe profile on the workpiece that is being machined.
Also related to ECM are U.S. Pat. Nos. 6,250,340; 6,413,407; 7,192,260; 5,310,468; 5,244,548; 5,149,405 and 6,309,195.